complibs/ppl: remove unnecessary (and confusing) --enable-cxx configure switch.
PPL does not use the "--enable-cxx" configure switch at all; it's
possibly a cut-and-paste leftover from 'gmp.sh'. (PPL is written in C++
natively, so it doesn't make much sense to have to enable C++; GMP, on
the other hand, is written in C with an optional C++ wrapper.)
Signed-Off-By: Anthony Foiani <anthony.foiani@gmail.com>